Ryan Golant
@rgolant.bsky.social
PhD candidate in Astronomy at Columbia University || cancer survivor || science communicator || cat enthusiast || professional waster of time
Today I learned that the guy who invented the hydraulic press was also instrumental in the development of the flush toilet. The Duality of Man.
May 10, 2025 at 7:52 PM
Today I learned that the guy who invented the hydraulic press was also instrumental in the development of the flush toilet. The Duality of Man.
A phenomenon that we do fully understand is the propagation of Alfvie waves across the surface of my cat. By perturbing his fur perpendicular to his stripes, we excite oscillations that propagate along his stripes. The energy in these waves is then usually converted into bites and scratches.
May 5, 2025 at 12:38 AM
A phenomenon that we do fully understand is the propagation of Alfvie waves across the surface of my cat. By perturbing his fur perpendicular to his stripes, we excite oscillations that propagate along his stripes. The energy in these waves is then usually converted into bites and scratches.
Alfvén waves are thought to be especially important in our Sun. By carrying energy from the solar surface to the corona, Alfvén waves may be responsible for heating the corona to its anomalously high temperature and for launching the solar wind – two processes that we still don’t fully understand.
May 5, 2025 at 12:38 AM
Alfvén waves are thought to be especially important in our Sun. By carrying energy from the solar surface to the corona, Alfvén waves may be responsible for heating the corona to its anomalously high temperature and for launching the solar wind – two processes that we still don’t fully understand.
Therefore, if we perturb a straight magnetic field line in the perpendicular direction, the line will want to snap back to its stable equilibrium; the field line will oscillate, launching a wave – an Alfvén wave – along the field line, much like a wave propagating along a plucked guitar string.
May 5, 2025 at 12:38 AM
Therefore, if we perturb a straight magnetic field line in the perpendicular direction, the line will want to snap back to its stable equilibrium; the field line will oscillate, launching a wave – an Alfvén wave – along the field line, much like a wave propagating along a plucked guitar string.
Some of these MHD phenomena resemble waves or instabilities that one might find in a neutral fluid – like the magnetic Kelvin-Helmholtz instability (depicted below in a simulation of the solar wind interacting with Earth's atmosphere) – while others are wholly unique to plasmas.
May 5, 2025 at 12:38 AM
Some of these MHD phenomena resemble waves or instabilities that one might find in a neutral fluid – like the magnetic Kelvin-Helmholtz instability (depicted below in a simulation of the solar wind interacting with Earth's atmosphere) – while others are wholly unique to plasmas.
For example, if we gently tap the surface of a body of water at rest, wave-like ripples will radiate outwards along the surface. However, if we blow air parallel to the surface of the water, the surface will quickly wrinkle and contort due to the so-called Kelvin-Helmholtz instability.
May 5, 2025 at 12:38 AM
For example, if we gently tap the surface of a body of water at rest, wave-like ripples will radiate outwards along the surface. However, if we blow air parallel to the surface of the water, the surface will quickly wrinkle and contort due to the so-called Kelvin-Helmholtz instability.
It’s important to note that not all equilibria are stable – consider a ball at rest on top of a sharp peak. Perturbations to these equilibria don’t induce waves, but instead trigger runaway processes called “instabilities” (to be explored further in future threads).
May 5, 2025 at 12:38 AM
It’s important to note that not all equilibria are stable – consider a ball at rest on top of a sharp peak. Perturbations to these equilibria don’t induce waves, but instead trigger runaway processes called “instabilities” (to be explored further in future threads).
Picture a ball at rest at the bottom of a valley. This ball is said to be in stable equilibrium: if we push this ball a little bit up the hill and then let it roll back (or, in physics speak, if we perturb this ball from its equilibrium), it’ll eventually return to rest at the bottom of the valley.
May 5, 2025 at 12:38 AM
Picture a ball at rest at the bottom of a valley. This ball is said to be in stable equilibrium: if we push this ball a little bit up the hill and then let it roll back (or, in physics speak, if we perturb this ball from its equilibrium), it’ll eventually return to rest at the bottom of the valley.
This is my cat, Alfvie. When I jiggle his belly, ripples propagate across his body. Similar phenomena occur in fluids and plasmas.
A thread on Alfvén waves [12 posts]:
#astronomy #astroedu #cats
A thread on Alfvén waves [12 posts]:
#astronomy #astroedu #cats
May 5, 2025 at 12:38 AM
This is my cat, Alfvie. When I jiggle his belly, ripples propagate across his body. Similar phenomena occur in fluids and plasmas.
A thread on Alfvén waves [12 posts]:
#astronomy #astroedu #cats
A thread on Alfvén waves [12 posts]:
#astronomy #astroedu #cats
To conclude: how does Alfvén’s theorem apply to my cat? Just as magnetic field lines are frozen into perfectly conducting plasmas, my cat’s stripes are frozen into his perfectly fluffy fur. This phenomenon – Alfvie's theorem – has been readily reproduced in laboratory experiments (see photos below).
April 28, 2025 at 12:18 AM
To conclude: how does Alfvén’s theorem apply to my cat? Just as magnetic field lines are frozen into perfectly conducting plasmas, my cat’s stripes are frozen into his perfectly fluffy fur. This phenomenon – Alfvie's theorem – has been readily reproduced in laboratory experiments (see photos below).
However, these clouds are threaded by magnetic field lines that are (nearly) frozen into the plasma. As the cloud spins and collapses, Alfvén’s theorem tells us that the plasma will try to twist up the field lines, but the tension in the field lines will resist this twisting.
April 28, 2025 at 12:18 AM
However, these clouds are threaded by magnetic field lines that are (nearly) frozen into the plasma. As the cloud spins and collapses, Alfvén’s theorem tells us that the plasma will try to twist up the field lines, but the tension in the field lines will resist this twisting.
For example, stars form when massive, spinning clouds of plasma collapse in on themselves. In the absence of a magnetic field, a cloud would spin faster and faster as it collapses, eventually yielding a centrifugal force that would tear the forming star apart.
April 28, 2025 at 12:18 AM
For example, stars form when massive, spinning clouds of plasma collapse in on themselves. In the absence of a magnetic field, a cloud would spin faster and faster as it collapses, eventually yielding a centrifugal force that would tear the forming star apart.
Alfvén’s theorem states that, if a plasma conducts electricity perfectly (often a good approximation in astrophysical systems), then magnetic field lines are “frozen into” the plasma. If the plasma moves, the field lines move with it; if the field lines move, the plasma is dragged along, too.
April 28, 2025 at 12:18 AM
Alfvén’s theorem states that, if a plasma conducts electricity perfectly (often a good approximation in astrophysical systems), then magnetic field lines are “frozen into” the plasma. If the plasma moves, the field lines move with it; if the field lines move, the plasma is dragged along, too.
As such, it’s not too far-fetched to picture magnetic field lines in MHD as tensile rubber ropes threading fluid-like plasmas; as a plasma flows, it tugs on the field lines and the field lines tug back.
April 28, 2025 at 12:18 AM
As such, it’s not too far-fetched to picture magnetic field lines in MHD as tensile rubber ropes threading fluid-like plasmas; as a plasma flows, it tugs on the field lines and the field lines tug back.
In many cases, magnetic field lines are just useful visual aids; the geometry of the field lines gives us a sense of the global structure of the field, and the density of the field lines tells us where the field is stronger or weaker (regions where lines are closer together have a stronger field).
April 28, 2025 at 12:18 AM
In many cases, magnetic field lines are just useful visual aids; the geometry of the field lines gives us a sense of the global structure of the field, and the density of the field lines tells us where the field is stronger or weaker (regions where lines are closer together have a stronger field).
To represent a magnetic field visually, one can draw arrows (or vectors, in math speak) at various points in space to indicate the local direction of the field; however, this can be cumbersome. Oftentimes, it’s more convenient to play connect-the-dots and trace out *magnetic field lines* instead.
April 28, 2025 at 12:18 AM
To represent a magnetic field visually, one can draw arrows (or vectors, in math speak) at various points in space to indicate the local direction of the field; however, this can be cumbersome. Oftentimes, it’s more convenient to play connect-the-dots and trace out *magnetic field lines* instead.
For a charged particle traveling through a magnetic field, the local direction and strength of the field (along with the particle’s charge and velocity) determine the direction and severity of the particle’s deflection.
April 28, 2025 at 12:18 AM
For a charged particle traveling through a magnetic field, the local direction and strength of the field (along with the particle’s charge and velocity) determine the direction and severity of the particle’s deflection.
First: what is a magnetic field? In broad terms, a magnetic field is a force field that deflects electrically charged particles. More precisely, a magnetic field is a vector field: at every point in space, the field has both a strength and a direction.
April 28, 2025 at 12:18 AM
First: what is a magnetic field? In broad terms, a magnetic field is a force field that deflects electrically charged particles. More precisely, a magnetic field is a vector field: at every point in space, the field has both a strength and a direction.
This is my cat, Alfvie. In my last thread, I mentioned that he’s very cute – this is still true. But how are Alfvie’s stripes like the magnetic fields in a plasma?
A thread on Alfvén’s theorem [15 posts]:
#astronomy #astroedu #cats
A thread on Alfvén’s theorem [15 posts]:
#astronomy #astroedu #cats
April 28, 2025 at 12:18 AM
This is my cat, Alfvie. In my last thread, I mentioned that he’s very cute – this is still true. But how are Alfvie’s stripes like the magnetic fields in a plasma?
A thread on Alfvén’s theorem [15 posts]:
#astronomy #astroedu #cats
A thread on Alfvén’s theorem [15 posts]:
#astronomy #astroedu #cats
While Hannes may not have been as cute as my cat, I still consider him a worthy namesake. In fact, the connections between Alfvie and Alfvén go even deeper – stay tuned for future threads!
April 20, 2025 at 7:51 PM
While Hannes may not have been as cute as my cat, I still consider him a worthy namesake. In fact, the connections between Alfvie and Alfvén go even deeper – stay tuned for future threads!
MHD, like hydrodynamics, is an approximation. However, it happens to be quite accurate at very large (i.e., astronomical) scales. In this way, Alfvén’s work paved the way for the study of astrophysical magnetic fields, such as those that permeate stars or wrap around galaxies.
April 20, 2025 at 7:51 PM
MHD, like hydrodynamics, is an approximation. However, it happens to be quite accurate at very large (i.e., astronomical) scales. In this way, Alfvén’s work paved the way for the study of astrophysical magnetic fields, such as those that permeate stars or wrap around galaxies.
The fundamentals of MHD were developed by the Swedish physicist and electrical engineer Hannes Alfvén (pictured below) in the mid-20th century, netting him the 1970 Nobel Prize in Physics.
April 20, 2025 at 7:51 PM
The fundamentals of MHD were developed by the Swedish physicist and electrical engineer Hannes Alfvén (pictured below) in the mid-20th century, netting him the 1970 Nobel Prize in Physics.
It’s not unreasonable to describe a plasma as a “gas” of electrically charged particles; while the air we breathe consists of many electrically neutral molecules, the plasma that comprises, say, our Sun, consists of many positively charged ions and negatively charged electrons.
April 20, 2025 at 7:51 PM
It’s not unreasonable to describe a plasma as a “gas” of electrically charged particles; while the air we breathe consists of many electrically neutral molecules, the plasma that comprises, say, our Sun, consists of many positively charged ions and negatively charged electrons.
At the microscopic level, gases and liquids consist of many tiny particles – atoms and molecules – whizzing around and bumping into one another; this microscopic behavior sets the properties of the larger system.
April 20, 2025 at 7:51 PM
At the microscopic level, gases and liquids consist of many tiny particles – atoms and molecules – whizzing around and bumping into one another; this microscopic behavior sets the properties of the larger system.
This is my cat, Alfvie. He’s very cute. But why did I name him Alfvie?
A thread on magnetohydrodynamics:
#astronomy #astroedu #cats
A thread on magnetohydrodynamics:
#astronomy #astroedu #cats
April 20, 2025 at 7:51 PM
This is my cat, Alfvie. He’s very cute. But why did I name him Alfvie?
A thread on magnetohydrodynamics:
#astronomy #astroedu #cats
A thread on magnetohydrodynamics:
#astronomy #astroedu #cats